The term drill cuttings as used herein refers to any material produced during drilling of a wellbore. For example, drill cuttings produced during drilling of a wellbore to access a subsurface hydrocarbon reservoir may typically comprise 70% solid matter, 15% water, and 15% oil. In the past, any drill cuttings produced offshore were simply dumped into the sea. However, this is now only permissible if the oil content of the drill cuttings is very low, typically less than 0.5% by mass. Accordingly, for the most part, drill cuttings produced offshore are stored on the drilling platform and then periodically shipped to shore for treatment. Such practices are, however, inefficient in terms of space utilisation, energy and cost.
Drill cuttings are brought to the surface in the drilling fluid. These fluids are then passed over a vibratory screen/shale shacker, to remove the solids which are in the form of an oil impregnated waste. The solids content can vary greatly depending on the formation being drilled through but are generally clay stone, silica or shale sands.
Currently the drill cuttings are stored in 5 tonne to 10 tonne skips which are generally called drill cuttings boxes (DCB). There can be 200 to 300 DCB required to drill a well and they are used to transport the drill cuttings back to shore for processing. More recently bulk tanks have been used to transport the cuttings to shore such a system is the Halliburton HCB Bulk System as designed and patented by William Curie. The bulk system eliminates 2000 offshore crane operations when drilling one well.
More recently, a system for processing drill cuttings has been used, which relies on mechanical manipulation of the drill cuttings to heat the drill cuttings for the purposes of separation of the oil and water from the solid matter. Such known processing systems typically utilise an electric motor for the mechanical manipulation. However, a processing system comprising a 750 kW electric motor may only have a throughput of approximately 2 to 3 tons of drill cuttings per hour and it would be desirable to increase the throughput or at least reduce the energy expended in processing the drill cuttings using such methods.
Accordingly there is a readily identifiable need for an effective method of treating the drill cuttings on the drilling rig so as to eliminate the need to transport drill cuttings to shore. Ideally if the oil on the cuttings can be reduced to less than 0.5% the solids can then simply be disposed of back in to the sea. The base oil can be reclaimed and reused.